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.TH LIST 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
LIST_EMPTY,
LIST_ENTRY,
LIST_FIRST,
LIST_FOREACH,
.\"LIST_FOREACH_FROM,
.\"LIST_FOREACH_SAFE,
.\"LIST_FOREACH_FROM_SAFE,
LIST_HEAD,
LIST_HEAD_INITIALIZER,
LIST_INIT,
LIST_INSERT_AFTER,
LIST_INSERT_BEFORE,
LIST_INSERT_HEAD,
LIST_NEXT,
.\"LIST_PREV,
LIST_REMOVE
.\"LIST_SWAP
\- implementation of a doubly linked list
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>
.PP
.B LIST_ENTRY(TYPE);
.PP
.B LIST_HEAD(HEADNAME, TYPE);
.BI "LIST_HEAD LIST_HEAD_INITIALIZER(LIST_HEAD " head );
.BI "void LIST_INIT(LIST_HEAD *" head );
.PP
.BI "int LIST_EMPTY(LIST_HEAD *" head );
.PP
.BI "void LIST_INSERT_HEAD(LIST_HEAD *" head ,
.BI "                        struct TYPE *" elm ", LIST_ENTRY " NAME );
.BI "void LIST_INSERT_BEFORE(struct TYPE *" listelm ,
.BI "                        struct TYPE *" elm ", LIST_ENTRY " NAME );
.BI "void LIST_INSERT_AFTER(struct TYPE *" listelm ,
.BI "                        struct TYPE *" elm ", LIST_ENTRY " NAME );
.PP
.BI "struct TYPE *LIST_FIRST(LIST_HEAD *" head );
.\" .BI "struct TYPE *LIST_PREV(struct TYPE *" elm ", LIST_HEAD *" head ,
.\" .BI "                        struct TYPE, LIST_ENTRY " NAME );
.BI "struct TYPE *LIST_NEXT(struct TYPE *" elm ", LIST_ENTRY " NAME );
.PP
.BI "LIST_FOREACH(struct TYPE *" var ", LIST_HEAD *" head ", LIST_ENTRY " NAME );
.\" .BI "LIST_FOREACH_FROM(struct TYPE *" var ", LIST_HEAD *" head ", LIST_ENTRY " NAME );
.\" .PP
.\" .BI "LIST_FOREACH_SAFE(struct TYPE *" var ", LIST_HEAD *" head ,
.\" .BI "                        LIST_ENTRY " NAME ", struct TYPE *" temp_var );
.\" .BI "LIST_FOREACH_FROM_SAFE(struct TYPE *" var ", LIST_HEAD *" head ,
.\" .BI "                        LIST_ENTRY " NAME ", struct TYPE *" temp_var );
.PP
.BI "void LIST_REMOVE(struct TYPE *" elm ", LIST_ENTRY " NAME );
.\" .PP
.\" .BI "void LIST_SWAP(LIST_HEAD *" head1 ", LIST_HEAD *" head2 ,
.\" .BI "                        struct TYPE, LIST_ENTRY " NAME );
.fi
.SH DESCRIPTION
These macros define and operate on doubly linked lists.
.PP
In the macro definitions,
.I TYPE
is the name of a user-defined structure,
that must contain a field of type
.IR LIST_ENTRY ,
named
.IR NAME .
The argument
.IR HEADNAME
is the name of a user-defined structure
that must be declared using the macro
.BR LIST_HEAD ().
.SS Creation
A list is headed by a structure defined by the
.BR LIST_HEAD ()
macro.
This structure contains a single pointer to the first element on the list.
The elements are doubly linked
so that an arbitrary element can be removed without traversing the list.
New elements can be added to the list
after an existing element,
before an existing element,
or at the head of the list.
A
.I LIST_HEAD
structure is declared as follows:
.PP
.in +4
.EX
LIST_HEAD(HEADNAME, TYPE) head;
.EE
.in
.PP
where
.I struct HEADNAME
is the structure to be defined, and
.I struct TYPE
is the type of the elements to be linked into the list.
A pointer to the head of the list can later be declared as:
.PP
.in +4
.EX
struct HEADNAME *headp;
.EE
.in
.PP
(The names
.I head
and
.I headp
are user selectable.)
.PP
.BR LIST_ENTRY ()
declares a structure that connects the elements in the list.
.PP
.BR LIST_HEAD_INITIALIZER ()
evaluates to an initializer for the list
.IR head .
.PP
.BR LIST_INIT ()
initializes the list referenced by
.IR head .
.PP
.BR LIST_EMPTY ()
evaluates to true if there are no elements in the list.
.SS Insertion
.BR LIST_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the list.
.PP
.BR LIST_INSERT_BEFORE ()
inserts the new element
.I elm
before the element
.IR listelm .
.PP
.BR LIST_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.SS Traversal
.BR LIST_FIRST ()
returns the first element in the list, or NULL if the list is empty.
.\" .PP
.\" .BR LIST_PREV ()
.\" returns the previous element in the list, or NULL if this is the first.
.\" List
.\" .I head
.\" must contain element
.\" .IR elm .
.PP
.BR LIST_NEXT ()
returns the next element in the list, or NULL if this is the last.
.PP
.BR LIST_FOREACH ()
traverses the list referenced by
.I head
in the forward direction,
assigning each element in turn to
.IR var .
.\" .PP
.\" .BR LIST_FOREACH_FROM ()
.\" behaves identically to
.\" .BR LIST_FOREACH ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found LIST element and begins the loop at
.\" .I var
.\" instead of the first element in the LIST referenced by
.\" .IR head .
.\" .PP
.\" .BR LIST_FOREACH_SAFE ()
.\" traverses the list referenced by
.\" .I head
.\" in the forward direction, assigning each element in turn to
.\" .IR var .
.\" However, unlike
.\" .BR LIST_FOREACH ()
.\" here it is permitted to both remove
.\" .I var
.\" as well as free it from within the loop safely without interfering with the
.\" traversal.
.\" .PP
.\" .BR LIST_FOREACH_FROM_SAFE ()
.\" behaves identically to
.\" .BR LIST_FOREACH_SAFE ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found LIST element and begins the loop at
.\" .I var
.\" instead of the first element in the LIST referenced by
.\" .IR head .
.SS Removal
.BR LIST_REMOVE ()
removes the element
.I elm
from the list.
.\" .SS Other features
.\" .BR LIST_SWAP ()
.\" swaps the contents of
.\" .I head1
.\" and
.\" .IR head2 .
.SH RETURN VALUE
.BR LIST_EMPTY ()
returns nonzero if the list is empty,
and zero if the list contains at least one entry.
.PP
.BR LIST_FIRST (),
and
.BR LIST_NEXT ()
return a pointer to the first or next
.I TYPE
structure, respectively.
.PP
.BR LIST_HEAD_INITIALIZER ()
returns an initializer that can be assigned to the list
.IR head .
.SH CONFORMING TO
Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008.
Present on the BSDs
(LIST macros first appeared in 4.4BSD).
.SH BUGS
.BR LIST_FOREACH ()
doesn't allow
.I var
to be removed or freed within the loop,
as it would interfere with the traversal.
.BR LIST_FOREACH_SAFE (),
which is present on the BSDs but is not present in glibc,
fixes this limitation by allowing
.I var
to safely be removed from the list and freed from within the loop
without interfering with the traversal.
.SH EXAMPLES
.EX
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>

struct entry {
    int data;
    LIST_ENTRY(entry) entries;              /* List */
};

LIST_HEAD(listhead, entry);

int
main(void)
{
    struct entry *n1, *n2, *n3, *np;
    struct listhead head;                   /* List head */
    int i;

    LIST_INIT(&head);                       /* Initialize the list */

    n1 = malloc(sizeof(struct entry));      /* Insert at the head */
    LIST_INSERT_HEAD(&head, n1, entries);

    n2 = malloc(sizeof(struct entry));      /* Insert after */
    LIST_INSERT_AFTER(n1, n2, entries);

    n3 = malloc(sizeof(struct entry));      /* Insert before */
    LIST_INSERT_BEFORE(n2, n3, entries);

    i = 0;                                  /* Forward traversal */
    LIST_FOREACH(np, &head, entries)
        np\->data = i++;

    LIST_REMOVE(n2, entries);               /* Deletion */
    free(n2);
                                            /* Forward traversal */
    LIST_FOREACH(np, &head, entries)
        printf("%i\en", np\->data);
                                            /* List deletion */
    n1 = LIST_FIRST(&head);
    while (n1 != NULL) {
        n2 = LIST_NEXT(n1, entries);
        free(n1);
        n1 = n2;
    }
    LIST_INIT(&head);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR insque (3),
.BR queue (7)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
